Did the speech therapist have any suggestions? It seems like she/he would have the most experience with kids who have dealt with this specific problem.
Is he better one-on-one or in a small group? It seems like some social skills practice in a more controlled environment would be helpful. (in a small group or with one or two kids at your house.) I also wonder if the excitement of big groups is adding overstimulation to the mix and he's a little overwhelmed by everything that's going on.
Does he sign at all? I am sure you want him to practice his speech as much as possible, but it seems like in limited-communication situations like the park with strangers it might be helpful for him to have another way to communicate. (not that random kids will necessarily know sign language, but he could use gestures for many things - and maybe the other kids' parents would get a hint right away that they might need to keep an eye on things.)
Anyway, based on experience with my son (ADHD and sensory integration issues at that age) I would try to avoid the situations that are going to be overwhelming or that provoke the problems - he will be a kid for a long time, no need to rush things! Find a quieter time of day to use the playground, for example, where there are fewer kids around. If he's not in preschool, see if your school district can offer anything; maybe they have an inclusion preschool group where special needs and typical children are all included and teachers are trained to help kids with social situations.
Best wishes to you! I think this age can be really hard for all kids and parents. They are able to do a lot more, but the judgment just isn't there yet for a lot of kids and they still spend a lot of time in their own fantasy worlds.